This invention relates generally to halogen lamps, and more particularly to a lamp vessel having a multi-layer optical interference coating for reflecting infrared radiation. More particularly, the coating includes alternating first and second layers, wherein the first layer has a relatively low refractive index, such as silica, and the second layer has a relatively high refractive index, such as tantala or niobia.
Interference filters having alternating layers of two or more materials of different refractive index are well known. Such coating films are used to transmit some wavelengths of radiation and to reflect other wavelengths. Such filters can be used to improve the illumination efficiency or efficacy of incandescent lamps or discharge lamps by reflecting infrared radiation emitted by an electric element, which may be a filament or a gas discharge, back to the element while transmitting the visible light. This lowers the amount of electrical energy required to maintain the operating temperature of the element.
Optical interference coatings or filters used for applications where the filter will be exposed to high temperatures, in excess of 500.degree. C., have been made of alternating layers of refractory metal oxides such as silica (SiO.sub.2)and one of tantala (Ta.sub.2 O.sub.5) and niobia (Nb.sub.2 O.sub.5). The silica is the low refractive index material (n=1.46) and the tantala (n=2.13) or niobia (n=2.35) is the high refractive index material. In such lamp applications, the filters are applied on the outside surface of the vitreous lamp envelope containing the filament or arc within and often reach operating temperatures as high as 900.degree. C. These interference filters are generally applied using chemical vapor deposition (CVD) and low pressure chemical vapor deposition (LPCVD) processes or reactive sputtering.
U.S. Pat. No. 4,663,557 to Martin et al. discloses an optical interference coating having alternating layers of SiO.sub.2 and Ta.sub.2 O.sub.5 which is suitable for use in high temperature environments. A problem with Ta.sub.2 O.sub.5, however, is that it crystallizes to a polycrystalline form at temperatures in excess of 800.degree. C., which causes the filter to scatter transmitted light and the reflected radiation.
U.S. Pat. No. 4,734,614 to Kuus describes a halogen lamp with an interference filter of SiO.sub.2 and Nb.sub.2 O.sub.5, the purpose of which is to overcome scatter and high temperature problems found with coatings of SiO.sub.2 /Ta.sub.2 O.sub.5. These coatings have been used successfully on 60 watt hard glass halogen burners that operate under 500.degree. C. and are placed in reflectors having lenses that are not hermetically sealed. However, 100 watt burners operate at high temperatures and require quartz glass lamp vessels in a hermetically sealed inert gas environment in order to protect the pinch area from oxidation.
Niobia coatings have the disadvantage that such coatings blacken when operated in an inert environment, such as in lamps where the reflector is hermetically sealed with a flame sealing process. This absorbing characteristic is apparently due to reduction of the niobia so that it is substoichiometric and therefore opaque. Tantala exhibits this problem to a much smaller degree, but tantalum sputtering targets are twice as expensive as niobium. Furthermore, the index of refraction for niobia is higher than for tantala (2.35 vs. 2.14), which can result in a thinner coating or higher efficiency.
In view of the foregoing it would be desirable to attain the advantages of tantala, e.g. limited tendency to become absorptive in an inert environment, and niobia, e.g. no high temperature scatter, lower cost, and high refractive index.